This invention relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hot gas flow path through a gas turbine engine. More particularly, this invention is directed to a coating system that exhibits improved high temperature corrosion resistance when used to protect a silicon-containing substrate.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. While nickel, cobalt and iron-base superalloys have found wide use for components throughout gas turbine engines, alternative materials have been proposed. In particular, silicon-based non-oxide ceramics, most notably with silicon carbide (SiC) and silicon nitride (Si3N4) as a matrix and/or reinforcing material, are candidates for high temperature applications, such as combustor liners, vanes, shrouds, airfoils, and other hot section components of gas turbine engines. However, when exposed to water-containing high temperatures such as that of a gas turbine engine, components formed of Si-based ceramics lose mass and recede because of the formation of volatile silicon hydroxide (Si(OH)4). The recession rate due to volatilization or corrosion is sufficiently high in a gas turbine engine environment to require an environmentally protective coating, commonly referred to as an environmental barrier coating (EBC).
Critical requirements for an EBC intended to protect gas turbine engine components formed of a Si-based material include stability, low thermal conductivity, a coefficient of thermal expansion (CTE) compatible with the Si-based ceramic material, low permeability to oxidants, and chemical compatibility with the Si-based material and a silica scale that forms from oxidation. Silicates, and particularly barium-strontium-aluminosilicates (BSAS; (Ba1-xSrx)O—Al2O3—SiO2) and other alkaline earth aluminosilicates, have been proposed as EBC's in view of their environmental protection properties and low thermal conductivity. For example, U.S. Pat. Nos. 6,254,935, 6,352,790, 6,365,288, 6,387,456, and 6,410,148 to Eaton et al. disclose the use of BSAS and alkaline earth aluminosilicates as outer protective coatings for Si-based substrates, with stoichiometric BSAS (molar ratio: 0.75BaO.0.25SrO.Al2O3.2SiO2; molar percent: 18.75BaO.6.25SrO.25Al2O3.50SiO2) generally being the preferred alkaline earth aluminosilicate composition. The use of rare earth silicates in EBC's has also been proposed, as taught in U.S. Pat. No. 6,759,151 to Lee. Layers of silicon, mullite (3Al2O3.2SiO2), and mixtures of mullite and BSAS have been proposed as bond coats to promote adhesion and limit reactions between an EBC and an underlying Si-based substrate. Commonly-assigned U.S. Pat. No. 6,844,075 to Saak et al. discloses the use of an intermediate layer of tantalum or niobium aluminate to inhibit solid-state reactions between a silicon-containing bond coat and an outer BSAS coating. Commonly-assigned U.S. Patent Application Publication No. 2005/0074625 to Meschter et al. teaches that additional aluminates can be used as an intermediate layer between a silicon-containing bond coat and an outer BSAS coating, as well as used an outer coating over a BSAS intermediate layer. If the particular component will be subjected to surface temperatures in excess of about 2500° F. (about 1370° C.), an EBC can be cooled with backside cooling of the substrate and thermally protected with an overlying thermal barrier coating (TBC) in accordance with commonly-assigned U.S. Pat. No. 5,985,470 to Spitsberg et al. In combination, these layers form what has been referred to as a thermal/environmental barrier coating (T/EBC) system.
The most commonly used TBC material for gas turbine applications is yttria-stabilized zirconia (YSZ). While exhibiting a desirable combination of properties, including low thermal conductivity, stability, good mechanical properties, and wear resistance, YSZ has a CTE mismatch with BSAS (a CTE of about 8.9-10.6 ppm/° C. for YSZ, compared to about 5.3 ppm/° C. for stoichiometric BSAS). YSZ also reacts with BSAS at temperatures greater than about 2500° F. (about 1370° C.), leading to sintering of the YSZ and a consequent loss in thermal and mechanical properties, resulting in through-thickness and horizontal cracking. To abate these problems, transition layers have been proposed containing mixtures of YSZ with alumina, mullite, and/or an alkaline earth metal aluminosilicate, as taught in commonly-assigned U.S. Pat. No. 6,444,335 to Wang et al.
While T/EBC systems as described above have significantly advanced the capability of using Si-based ceramic materials for high temperature components, further improvements would be desirable. In particular, the above-noted silicates, including BSAS and mullite, have been observed to be susceptible to corrosion promoted by contaminants such as sea salt and CMAS, the latter of which is a eutectic composition of calcia, magnesia, alumina and silica found as contaminants within a gas turbine engine during operation. Sea salt and CMAS are able to infiltrate porous TBC's, especially those deposited to have a columnar microstructure as a result of being deposited by physical vapor deposition (PVD), with the result that silicates within the transition layer and the EBC are both subject to corrosive attack. Furthermore, at temperatures exceeding about 2800° F. (about 1540° C.), BSAS and mullite-containing EBC's and transition layers have been observed to react with YSZ-containing transition layers and TBC's as a result of silica from BSAS and/or mullite gettering yttria from the YSZ-containing transition layer or TBC to form Y2SiO5, which eventually leads to spallation of the TBC and failure of the T/EBC system.